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In diastole, left ventricular pressure continues to drop and falls below left atrial pressure. The mitral valve opens. This is usually a silent event, but may be audible as a pathologic opening snap (OS) if valve leaflet motion is restricted, as in mitral stenosis.

Aortic pressure

Mitral valve opens

Left atrial pressure Left ventricular pressure

S2 OS

Aortic pressure

Mitral valve opens

Left atrial pressure Left ventricular pressure

Si Ej

S2 OS

After the mitral valve opens, there is a period of rapid ventricular filling as blood flows early in diastole from left atrium to left ventricle. In children and young adults, a third heart sound, S3, may arise from rapid deceleration of the column of blood against the ventricular wall. In older adults, an S3, sometimes termed "an S3 gallop," usually indicates a pathologic change in ventricular compliance.

Period of rapid ventricular filling

S1 Ej S2 OS S3

Period of rapid ventricular filling

S1 Ej S2 OS S3

Finally, although not often heard in normal adults, a fourth heart sound, S4, marks atrial contraction. It immediately precedes S1 of the next beat, and also reflects a pathologic change in ventricular compliance.

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